INTERLAYER INTERACTION AND SCREENING IN MoS2

Electronic band structures and phonon bands of bulk, bilayer and monolayer MoS 2 are studied by DFT and GW methods and discussed in the framework of applicability of the 2D model for layered systems. The MoS 2 monolayer is found to be a direct gap semiconductor with the top of the valence band and the bottom of the conduction band at K-point. For a bulk MoS 2, the bandgap, estimated within GW, agrees with the indirect bandgap obtained in experiment. However, for a MoS 2 monolayer, the GW-derived gap exceeds the energy of the photoluminescence line by ~0.9 eV. In contrast, the LDA value (1.84 eV) is consistent with the position of the photoluminescence line as well as with absorption spectroscopy and photoconductivity experiments. A significant overlap of the LDA-derived electron densities (in contrast to GGA results) indicates that the interlayer interaction in MoS 2 can be explained by the exchange interaction. Phonon bands, calculated for MoS 2 monolayer and bilayer within LDA, are consistent with Raman spectra, thus indicating the capability of LDA to correctly describe interactions in the layered system, including the interlayer interactions usually attributed to van der Waals forces.

Exciton Liquid in Coupled Quantum Wells

Excitons in semiconductors may form correlated phases at low temperatures. We report the observation of an exciton liquid in gallium arsenide/aluminum gallium arsenide–coupled quantum wells. Above a critical density and below a critical temperature, the photogenerated electrons and holes separate into two phases: an electron-hole plasma and an exciton liquid, with a clear sharp boundary between them. The two phases are characterized by distinct photoluminescence spectra and by different electrical conductance. The liquid phase is formed by the repulsive interaction between the dipolar excitons and exhibits a short-range order, which is manifested in the photoluminescence line shape.

Ultraviolet Radiation Action onto Nanostructures with Quantum Wells GaAs/AlGaAs

Optical properties modification of semiconductor heterostructure with quantum wells GaAs/AlGaAs by the ultraviolet (UV) radiation was investigated in the range of energy density of 0 – 500 mJ/cm2 . It was shown that photoluminescence signal amplitude decreases with an increase of UV energy density; this dependence revealed threshold nature with the threshold value about 100 mJ/cm2 – in agreement with other results on laser formation of the point defects in GaAs. Reflectivity kinetics of the sample detected at the probe radiation wavelength corresponding to the photoluminescence line center showed an increase of electron-hole recombination rate with growth of UV energy density. Sharp focusing of UV laser radiation led to a plume formation over the sample surface; spectrum of the plume contained neutral gallium atoms doublet at wavelengths of 403 нм and 417 nm